Re: device compatibility interface for live migration with assigned devices

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Alex Williamson (alex.williamson@redhat.com) wrote:
> On Mon, 27 Jul 2020 15:24:40 +0800
> Yan Zhao <yan.y.zhao@intel.com> wrote:
> 
> > > > As you indicate, the vendor driver is responsible for checking version
> > > > information embedded within the migration stream.  Therefore a
> > > > migration should fail early if the devices are incompatible.  Is it  
> > > but as I know, currently in VFIO migration protocol, we have no way to
> > > get vendor specific compatibility checking string in migration setup stage
> > > (i.e. .save_setup stage) before the device is set to _SAVING state.
> > > In this way, for devices who does not save device data in precopy stage,
> > > the migration compatibility checking is as late as in stop-and-copy
> > > stage, which is too late.
> > > do you think we need to add the getting/checking of vendor specific
> > > compatibility string early in save_setup stage?
> > >  
> > hi Alex,
> > after an offline discussion with Kevin, I realized that it may not be a
> > problem if migration compatibility check in vendor driver occurs late in
> > stop-and-copy phase for some devices, because if we report device
> > compatibility attributes clearly in an interface, the chances for
> > libvirt/openstack to make a wrong decision is little.
> 
> I think it would be wise for a vendor driver to implement a pre-copy
> phase, even if only to send version information and verify it at the
> target.  Deciding you have no device state to send during pre-copy does
> not mean your vendor driver needs to opt-out of the pre-copy phase
> entirely.  Please also note that pre-copy is at the user's discretion,
> we've defined that we can enter stop-and-copy at any point, including
> without a pre-copy phase, so I would recommend that vendor drivers
> validate compatibility at the start of both the pre-copy and the
> stop-and-copy phases.

That's quite curious; from a migration point of view I'd expect if you
did want to skip pre-copy, that you'd go through the motions of entering
it and then not saving any data and then going to stop-and-copy,
rather than having two flows.

Note that failing at a late stage of stop-and-copy is a pain; if you've
just spent an hour migrating your huge busy VM over, you're going to be
pretty annoyed when it goes pop near the end.

Dave

> > so, do you think we are now arriving at an agreement that we'll give up
> > the read-and-test scheme and start to defining one interface (perhaps in
> > json format), from which libvirt/openstack is able to parse and find out
> > compatibility list of a source mdev/physical device?
> 
> Based on the feedback we've received, the previously proposed interface
> is not viable.  I think there's agreement that the user needs to be
> able to parse and interpret the version information.  Using json seems
> viable, but I don't know if it's the best option.  Is there any
> precedent of markup strings returned via sysfs we could follow?
> 
> Your idea of having both a "self" object and an array of "compatible"
> objects is perhaps something we can build on, but we must not assume
> PCI devices at the root level of the object.  Providing both the
> mdev-type and the driver is a bit redundant, since the former includes
> the latter.  We can't have vendor specific versioning schemes though,
> ie. gvt-version. We need to agree on a common scheme and decide which
> fields the version is relative to, ex. just the mdev type?
> 
> I had also proposed fields that provide information to create a
> compatible type, for example to create a type_x2 device from a type_x1
> mdev type, they need to know to apply an aggregation attribute.  If we
> need to explicitly list every aggregation value and the resulting type,
> I think we run aground of what aggregation was trying to avoid anyway,
> so we might need to pick a language that defines variable substitution
> or some kind of tagging.  For example if we could define ${aggr} as an
> integer within a specified range, then we might be able to define a type
> relative to that value (type_x${aggr}) which requires an aggregation
> attribute using the same value.  I dunno, just spit balling.  Thanks,
> 
> Alex
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK